Make ds_ping_interval changeable at runtime

[neuhaus]

Feature request for master/slave setups:

What we would like to see is the ability to globally pause all OPTIONs
checks (set ds_ping_interval temporarily to 0 at runtime) -
or provide a different mechanism to temporarily disable pings.

When you have a master/slave Kamailio setup the slave will currently fail all its pings because
it has no network access and it fills its logfile with errors.

Also, it will mark the other hosts as down so when it takes over from the master it takes a while until it notices that the other hosts are in fact up.

[miconda]

Implemented a mechanism to disable keepalives via new rpc command -- see docs on master branch.

[neuhaus]

Thanks, I'll have a look.